1. Field of the Invention
The present invention generally relates to acoustic transducers and, in particular, to linear transducer arrays that can perform either or both pulse-echo and holographic acoustic imaging.
2. Description of the Prior Art
The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section XI, 1974 Edition, "Rules For the Inspection of Nuclear Power Plant Components" and the Summer 1974 Addenda sets down the requirements for volumetric inservice inspection of welds in nuclear reactor pressure vessels, piping, and nozzles. The code requires examination of 0.degree., 45.degree., and 60.degree. sound beam viewing from both sides of a weld. Current testing systems used to inspect pressure vessels from the outside surface employ quasi-contact techniques to couple the ultrasonic energy from a multiple transducer head into the surface of the component being tested. The transducer heads are moved across the weld and the data are recorded on magnetic tape, CRT display, pen recorder or a combination of displays. The systems are relatively slow and require several minutes to cover a one-foot section of weld. Interpretation of the recorded data requires detailed examination and is subject to human interpretation in order to measure the dimensions of the flaws found in the component.
Acoustic holography has been successfully used to characterize known defects; however, present single surface holographic techniques are quite slow. Typically these systems require between five to ten minutes to scan one square foot of weld. In addition, many holograms are required to develop an accurate interpretation of the ultrasonic reflector.
Ultrasonic arrays are currently being used with great success in the medical ultrasonic diagnostic field. These techniques primarily use the pulse-echo method of operation. The use of arrays has been proposed for industrial applications, but while several concepts have been researched, no validated system has yet to be developed. The use of ultrasonic arrays for holographic imaging has also been researched and validated, but the technology has not been carried to demonstration instrumentation.
Both pulse-echo and holographic techniques utilize acoustic waves that are directed into the material being tested and reflected from surface and subsurface defects. The energy returned from a defect is a function of its size, acoustic impedance, orientation, shape, and depth within the material. For a pulse-echo measurement the time delay and signal amplitude of the reflected acoustic waves are measured. In holographic imaging the time delay, phase and signal amplitude are used to describe the defect. It is also worthy to note that pulse transmission techniques are used in both pulse-echo and holographic displays.
For pulse-echo imaging the transmitted acoustic waves should be collimated into a defined and controlled beam of acoustic waves so that the waves can penetrate deep into the material being tested and return signal information about the size of the flaws. To achieve this effect, pulse-echo transducers are usually designed for directionality and have very small angles of acceptance to off-axis incident acoustic waves.
In contrast, holographic imaging requires the transmission of a dispersive acoustic wave. The receiving transducers are generally small in order to have a large angle of acceptance and non-directional sensitivity. The receiving transducers also must maintain the phase coherence of the reflected acoustic waves incident on the receiving transducer.
Heretofore, a successfully operable transducer array that can be used for both pulse-echo and holographic acoustic imaging has not been developed because of the seemingly mutually exclusive operating requirements outlined above. Although there are transducer arrays available today that can perform one of these scanning techniques separately, there is as yet no system that can successfully perform both pulse-echo and holographic acoustic imaging with the same linear array.